1. Field of the invention
The present invention relates to a heat-developable photosensitive material making use of an organic silver salt.
2. Related Background Art
Silver salt photography making use of a silver halide is a recording technique which has been hitherto widely put into use because of excellent sensitivity and gradation. In this photography, however, after imagewise exposure the processing including developing, stopping and fixing is carried out by a wet process, and hence its deficiencies in operability, simplicity and safety have been questioned. As a countermeasure therefor, research has been made on dry-process materials that do not require such wet processing, as disclosed in Japanese Patent Publication Nos. 43-4921, No. 43-4924, etc. These disclosures are concerned with a technique in which a photosensitive silver halide is used in an amount required as a catalyst and a non-photosensitive organic silver salt is used as an image forming agent. The mechanism by which the organic silver salt acts as an image forming agent can be explained as follows: (1) A silver nucleus is produced from a photosensitive silver halide as a result of exposure, and it forms a latent image. (2) The silver nucleus serves as a catalyst, an organic silver salt and a reducing agent cause oxidation-reduction reaction upon heating, and the organic silver salt is reduced to metallic silver, which forms a visible image.
As an example of methods of utilizing such a heat-developable photosensitive material, Japanese Patent Application Laid-open No. 55-50246 discloses a method of use as a mask. In this method, a silver image is used as a mask. As a photosensitive material capable of obtaining a polymer image with much better contrast than those utilizing the silver image mask, Japanese Patent Application Laid-open No. 3-135564 discloses a photosensitive material that utilizes light absorption of a light-absorbing organic compound that is an oxidized product of a reducing agent, to form an image with a better contrast.
Meanwhile, a protective layer used in the above dry-process materials is disclosed, for example, in Japanese Patent Publication No. 59-13728. This publication discloses an acrylic resin composition improved in impact strength. Japanese Patent Publication No. 2-4889 also discloses as a material for a topcoat polymer layer polyvinyl butyral, polystyrene, polymethyl methacrylate or polyurethane rubber.
However, in conventional heat-developable photosensitive materials, there has been the problem that the moisture in the air enters into a photosensitive layer through the protective layer to lower sensitivity, making it impossible to store such materials over a long period of time as unused materials. In addition, in conventional heat-developable photosensitive materials, the protective layer tends to be softened by the heat generated during heat development to cause scratches on the surface of the protective layer, or cause what is called bleeding wherein components in the photosensitive layer migrate to the surface of the protective layer in a powdery state, resulting in adhesion of a nonwoven fabric constituting a heating element to the protective layer surface. There also has been the problem that images undergo distortion because of an insufficient strength of the protective layer upon application of an external force to the photosensitive material when the protective layer is softened by the heat generated during heat development.